Electron image device



ELECTRON IMAGE DEVICE Filed Jan. 26, 1965 OUTPUT SECOND CROSSOVER /\.FIRST CROSSOVER TOTAL ELECTRONS EMITTED TOTAL ELECTRONS INCIDENT WITNESSES INVENTORS W10 w Rolf R. Beyer a Alvin H. Bee a ATTORNEY 3,431,455 Patented Mar. 4, 1969 8 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to an electron image device including a target member having a electrically conductive element or plate and a layer of an insulating storage material for storing a pattern of charges, suitable means such as an electron gun for directing a beam of electrons onto the surface of said layer of insulating material, and a grid spaced from the surface of the layer of insulating material. The grid is connected to the element by a sub stantially zero impedance path in order to prevent a field across the layer of insulating material of a magnitude to cause the breakdown of the layer.

This invention relates to an electron image device, and, more particularly, to those devices in which information may be recorded on a target member by incident radiation and then read-out by means of an electron beam.

In an electron image device as described in the copending application, Ser. No. 241,641 by Goetze and Boerio, now US. Patent No. 3,213,316, entitled, Image Storage System, and assigned to the assignee of this invention, an image is recorded by focusing high energy electrons upon a target member. The target member comprises a conductive electrode upon which has been dis posed a layer of insulating material capable of storing electron charges. The incident high energy electrons cause discrete portions of the insulating layer to become conductive, and as a result create a pattern of charges upon the surface of the insulating layer corresponding to that pattern of information conveyed by the high energy electrons. In order to detect or readout the recorded information, a low energy electron beam is directed upon the target member; as the electrons are attracted to the positive portions of the target member, a signal may be derived from the conductive back plate due to the simple capacitive coupling established across the insulating coating between those positive areas and the conductive electrode. This type of-read-out operation is similar to that of the well-known vidicon television camera.

Due to the complicated and erratic motion of the low energy electron beam, it may be understood that the low energy electron beam does not always fall normally upon the target member. In order to insure a precise and normal landing of the low-energy electrons, a grid electrode formed of a fine wire mesh is inserted between an electron gun, Which emits the low energy electrons, and the target member for collimating the electrons into a path substantially normal to the surface of the target member. Typically, a voltage in the range of 200 to 400 volts may be applied to the grid electrode. In the course of operation of this device, the surface of the insulating coat ing of the target member may be placed at a potential above the first crossover potential of the insulating material comprising the target member. As a result, the incident, low-energy electron beam will be attracted to the target member with suflicient velocity to emit secondary electron in numbers exceeding that of the incident lowenergy electrons. When this phenomena occurs, the emitted secondary electrons will be attracted by the high potential placed upon the grid electrode, and as a consequence, the surface of the insulating coating of the target member becomes increasingly positive due to the loss or subtraction of the electrons. This process will continue until the surface of the insulating coating assumes the potential of the grid inserted between the cathode gun and the target member. However, as noted before, the potential applied to the grid electrode may be in the order of 200 to 400 volts; if such a voltage were applied across the insulating coating of the target member, the resultant field would be sufiicient to cause the break-down and destruction of the insulating coating.

As disclosed in the above-mentioned, copending application, a second or auxiliary grid is inserted between the first grid electrode for collimating the low energy beam of electrons and the target member to substantially prevent the destruction of the insulating coating of the target member. A positive potential below the first cross-over of the insulating coating is applied to the auxiliary grid electrode. Therefore, it may be understood that the potential of the surface of the insulating coating may not exceed the potential imposed upon the auxiliary grid, and as a result the target member may not operate above the first cross-over of the insulating material and thus the destruction of the target member is prevented.

Though the insertion of an auxiliary grid substantially prevents the destruction of the insulating coating, two significant areas of difficulty were presented by the incorporation of such an auxiliary grid. First, in order to achieve an adequate resolution upon the target member, the intensity of the electrostatic field must be sufiicient to precisely direct the electrons upon the target member. However, the voltage placed upon the auxiliary grid must be below the first cross-over of the insulating material. Therefore, in order to establish the necessary electrostatic field, the auxiliary grid must be spaced very close (in the order of several mils) from the insulating coating of the target member. The spacing of such an auxiliary grid presents a diflicult fabrication problem in that the grid electrode must be precisely spaced from the insulating coating and must be electrically insulated therefrom. A second disadvantage resulting from the introduction of such an auxiliary grid resides in the undesired capacity (approximately 30 picofarads) presented by the auxil iary grid between the plate electrode of the target member and ground. Such capacity must be compensated for in the amplifier circuitry associated with the target member in order to maintain the required frequency response; as a consequence of such circuitry, the noise level of the entire image device would be increased.

A possible solution to these above-enumerated problems would be simply to eliminate the auxiliary grid. However, it would be necessary to constantly monitor the operation of the target member and to immediately discontinue operation when an intolerable large signal is reached. This method is not satisfactory because it does not assure absolute protection of the target member.

It is, accordingly, an object of this invention to provide an improved electron image device.

A further object of this invention is to provide a target member for an electron image device that can be operated with safety at all signal levels.

A still further object of this invention is to provide an electron image device wherein an auxiliary grid may be spaced with respect to the target member to ensure precise registration of the low energy electron beam upon the target member.

A still further object of this invention is to provide an electron image device wherein there is incorporated an auxiliary grid that requires no additional power supply to be associated therewith.

Another object of this invention is to provide an electron image device having a target member therein which permits the use of a closely spaced auxiliary grid without the introduction of shunt capacity and which substantially prevents the introduction of microphonics in the output of such a device.

Briefly, the objects of this invention are accomplished by providing an electron image device in which there has been incorporated an auxiliary grid or mesh in close proximity with a target member. More specifically, the auxiliary mesh has been electrically connected directly to the plate electrode of the target member.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this description.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is an elevational view in section, schematically representing an electron image device and associated system in accordance with the teachings of this invention;

FIG. 2 is a sectional view of a specific embodiment of the target member incorporated within the image device of FIG. 1;

FIG. 3 is a sectional view of an alternative embodiment of the target member incorporated within the image device of FIG. 1; and

FIG. 4 is a graph to assist in the explanation of this invention.

Referring in detail to the drawings and in particular to FIG. 1, there is illustrated an electron image device such as a television camera tube incorporating the teachings of this invention. The electron image device 10 comprises an envelope 12 made of a suitable insulating material and having one end thereof enclosed by a face I plate 14. The face plate 14 is designed to be transmissive to the radiation emanating from a scene 46 and is made of a suitable material such as glass in the case of a visible light input. A photocathode 15 is provided on the interior surface of the face plate 14 and is made of a photoemissive material sensitive to the input radiation such as cesium antimony for a visible light input. An electron gun 16 is provided at the opposite end of the envelope 12 for generating and forming a pencil-type electron beam which is directed upon a target member 26. The electron gun 16 is of any suitable type for producing a low velocity pencillike electron beam and may consist of a cathode element 18, a control grid 20, and an accelerating grid 22. The electrodes 18, 20 and 22 of the electron gun 16 along with a field electrode 24 formed as a coating upon the interior of the envelope 12 provide a focused electron beam which is directed upon the target member 26. Deflection means 42 illustrated as a coil is provided around the envelope 12 for deflecting the electron beam, and by application of suitable potential, the low energy electron beam is scanned over the surface of the target member 26 in a conventional manner. A focusing means 44 illustrated as a coil is also provided about the envelope 12 to provide focusing of the electron beam from the electron gun 16 onto the target member 26. In addition, the focusing means 44 also focuses the electrons emitted from the photocathode 15 onto the target member 26.

The target member 26 is disposed within the envelope 12 and between the electron gun 16 and the photocathode element 15. Between the target member 26 and the photocathode element 15, there are provided a plurality of electrodes illustrated as 38 and 40 with suitable potentials provided thereon for accelerating and focusing electrons emitted from the photocathode element 15 onto the target member 26. Positioned between the target member 26 and the electron gun 16, there is provided a first mesh or grid 34 adjacent to and parallel to the target member 26. The grid 34 is formed of a fine wire mesh made of an electrically conductive material such as nickel. Further, in one embodiment of the invention the grid 34 is placed approximately .050 inch from the surface of the target member 26 for collimating the electrons emitted by the electron gun 16 into a path substantially normal to the surface of the target member 26.

In an illustratve embodiment of this invention, the target member 26 is comprised of a support ring 28 made of a suitabe material such as Kovar (a trademark of the Westinghouse Electric Corporation for an alloy of nickel, iron and cobalt) having a suitable electrically conductive support plate or element 30 as aluminum attached there to. A coating or layer 32 of a suitable insulating or semiconducting material, which exhibits the property of generating electrons in response to the input of an energy radiation such as electrons, light or X-rays so as to produce a pattern of charges on the surface thereof in response to the input radiation, is provided on the electrically conductive support plate 30 facing the electron gun 16. The coating 32 may be of any suitable material such as an alkaline or alkaline earth metal compound such as potassium, magnesium chloride or magnesium oxide.

In order to supply a specific example, an illustrative embodiment of a suitable target member 26 will now be described. The aluminum plate 30 may be formed by the vacuum deposition of aluminum onto a film of thermally removable organic material such as cellulose nitrate. The thickness of the alumin um plate 30 should be about 1000 angstroms for an electrode diameter of approximately one inch. The cellulose nitrate is baked out leaving the aluminum plate 30 attached to the support ring 28. This technique is more fully described in US. Patent No. 2,905,844, assigned to the assignee of this invention. The plate 30 and the support ring 28 are then placed in a bell jar having an atmosphere of approximately 1 millimeter mercury of argon or any other inert gas. A boat of suitable material such as tantalum provided with a resistive heating element is positioned within the bell jar. A predetermined amount such as 16 milligrams of a suitable material such as potassium chloride is placed in the boat. The boat is then placed at a distance of approximately 3 inches below the aluminum plate 30 and current is applied to the resistive heating element of the boat. The heat is applied until the material has just melted at which temperature the material is then maintained. The vapor pressure of the material at its melting point under such conditions is found sufiicient to cause vaporization of the material at a sufiicient rate. The material is evaporated to completion and it is found that the density of the evaporated storage material on the aluminum layer is approximately 1 to 5 percent of its bulk density. The coating 32 has a thickness of approximately 20 microns.

It is a significant aspect of this invention that a second or auxiliary grid 36 be inserted between the first grid 34 and the target member 26. In an illustrative embodiment of this invention, the second grid 36 is comprised of a fine mesh having a transmission of about and being made of an electrically conductive material such as copper. It is particularly noted, that the second grid 36 is electrically connected as by a substantially zero impedance connection 37 to the plate 30 of the target member 26 so that both the plate 30 and the grid 36 are operated at the same AC. and DC. voltage conditions. As will be explained in greater detail, the breakdown of the insulating coating 32 is thereby prevented since the voltage applied to the grid 36 as by a potential source 39 may now be controlled to prevent the breakdown of the insulating coating 32. Further, the grid 36 is spaced about 0.005 inch from the target member 26 to provide a field of approximately 3 volts/mil and to ensure proper registration of the low energy electron beam upon the target member 26.

Referring now to FIG. 2, an illustrative embodiment of the manner in which the auxiliary grid 36 is mounted with respect to the target 26 is shown. The grid 36 may be secured as by spot welding to an annular support member 37:: made of a suitable electrically conductive material such as Kovar (a trademark of the Westinghouse Electric Corporation, supra). The support member not only serves the function of supporting the grid 36 in a plane closely spaced from and parallel to the insulating coating 32 but also serves the purpose of electrically connecting the grid 36 to the plate 3 0. Further, the support member 37a may be secured as by spot welding to a support ring 218.

Further with regard to FIG. 3, an alternative embodiment of the target is presented. The storage target 26a comprises a support layer 31a made of a suitable insulating material such an aluminum oxide. An electrically conductive back plate 30a made of a suitable material such as aluminum is deposited upon the support layer 31a and an insulating coating 32a is placed upon the plate 30a. In an illustrative method of manufacture, the support layer 31a is produced by first oxidizing a plate of aluminum and then etching the aluminum plate away to leave a layer 31a of aluminum oxide 1000 A. thick. Next, the plate 30a is formed by evaporating aluminum onto the support layer 31a to a depth of 1000 A. Finally, the insulation layer 32a may be formed by evaporating a porous layer of potassium chloride onto the plate 30a in the manner described above.

In order to more fully describe this invention, the operation of this invention will be set out with respect to the representative values of the potentials applied to the electrodes as illustrated in FIG. 1. Initially, a potential of approximately volts with respect to the cathode element 18 is applied to the plate 30 of the target member 26. The low energy electrons emitted by the electron gun 16 are focused as by the field electrode 24 and the focusing means 44 onto the surface of the insulating coating 32. As a result, the surface of the insulating coating 32 is stabilized or maintained at an equilibrium potential which may be substantially ground potential by means of the scanning electrons emitted from the electron gun 16. It is noted that in the illustrative embodiment presented in FIG. 1 the cathode element 18 of the electron gun 16 is connected to ground potential. Further, the first grid 34 is maintained at approximately a potential of 450 volts positive with respect to ground to thereby collimate the electrons emitted by the electron gun 16 into a path normal to the surface of the target member 26.

A light image from the scene 46 is focused upon the photocathode element 15 and photoelectrons are emitted from each portion of a photocathode element 15 corresponding to the amount of light directed thereon. The photocathode element 15 is operated at a potential of about 8000 volts negative with respect to the plate 30 to provide acceleration of electrons from the photocathode element 15. The photoelectrons emitted by the photocathode element 15 are focused upon the target member 26 as by the focusing means 44 and are accelerated to a sufficiently high energy by the accelerating electrodes 38 and 40 to which are respectively applied potentials of 3500 volts and 7000 volts negative with respect to the plate 30. The photoelectrons are accelerated to a sufficiently high energy of about 8000 electron volts so that they penetrate through the conductive plate 30 and enter into the insulating coating 32. The acceleration voltage should be adjusted such that substantially all of the primary electrons emitted from the photocathode element 15 almost completely penetrate the entire target member 26 but do not substantially pass on through the structure. For example, in the case of a target member 26 having a conductive plate 30 of aluminum with a thickness of about 1000 angstroms and a porous insulating coating 32 of a thickness of about microns the acceleration should be about 8000 volts. The electrons (i.e., primary electrons) emitted from the photocathode element 15 create as they penetrate the insulating coating 32 a certain number of low energy electrons within the coating 32 in orders of magnitude higher than the numher of incident or primary electrons; the number of secondary electrons generated may be about 200 for each primary electron penetrating the insulating coating 32. As explained before, the surface of the insulating coating 32 exposed to the electron gun 16 has been stabilized at approximately ground potential to thereby polarize or establish a field between the exposed surface of the insulating coating 32 and the electrically conductive plate 30 which has been set at a potential of approximately 15 volts with respect to ground. Thus, the low energy electrons generated within the insulating coating 32 cause the exposed surface to change its potential locally due to the conduction of the electrons across the coating 32 to the positive plate 30 and due to secondary electrons emitted from the exposed surface of coating 32 which are then collected by electrodes 36 and 34. By so creating a pattern of discrete changes of potential upon the exit surface of the coating 32, a video signal may be provided by using any of the several well-known read-out techniques. In FIG. 1, there is illustrated a typical vidicon type read-out assembly.

Referring now to FIG. 4, the curve shown therein is a general curve showing the reflective secondary emission ratio as a function of primary bombarding electron energy. The primary energy is expressed in electron volts. The secondary emission ratio is simply the ratio of the secondary electron emission current to the primary current. It is particularly noted that the secondary emission ratio is unity at two locations and these locations are designated as the first and second cross-over energies. Thus, it may be seen that at points below the first crossover, that the number of reflected secondary electrons from the surface of a material is less than the number of primary electrons and therefore the surface will tend to charge in a negative direction. On that portion of the curve between the first and second cross-over points, the energy of the primary electrons will be such as to create more reflective secondaries from the surface of the tar-get than primaries incident thereon. This results in the surface tending to charge in a positive direction. Further, for those points upon the curve above second cross-over, the number of emitted electrons will again be less than the number of primary electrons incident thereon, and therefore the surface will again tend to charge in a negative direction. It is noted that in the typical operation of the electron emission device 10 that the electrons emitted from the electron gun 16 are accelerated with energy below the first cross-over of the material of the coating 32.

In the operation of the target member 26, it may be initially assumed that the exit surface potential (V of the insulating coating 32 is at ground or at the gun cathode potential. Due to the bombardment of the target member 26 by electrons emitted from the photocathode element 15, secondary emission occurs from the exposed surface of the coating 32 and conduction of electrons is established through the coating 32. As a result, the surface potential V of the coating 32 drifts toward the potential V of the plate 30 due to the field applied across the coating 32. The conduction due to electrons Within the coating 32 becomes zero when a potential V of the exposed surface of the coating 32 is at the same potential as the plate 30. Secondary emission continues and drives the potential V past the target plate potential V which results in a reversal of polarity of electric field across the coating 32. It is believed that the continued charging of the exposed surface of the coating 32 is due to the excess of transmission secondary emission over the free electron conduction current within the coating 32. It is possible, in the absence of a low voltage auxiliary grid adjacent to the exit surface, for the potential of the exposed surface of the insulating coating 32 to charge positive with respect to the cathode element 18 to a point where the acceleration of the low energy reading beam of electrons onto the target member 26 will exceed the first cross-over potential of the material in the coating 32. In such a circumstance if the exposed surface of the coating 32 exceeds the first cross-over potential, then not only would the exposed surface of the coating 32 be charged positively due to the transmission secondary emission from the bombardment of the target member 26 by the beam of electrons emitted by the photocathode element 15, but also due to the scanning beam of electrons emitted by the cathode gun 16. This would result in the voltage V of the exposed surface of the coating 32 building up to such a value (i.e. breakdown voltage) so as to destroy the insulating or semiconductor coating 32 by impressing a strong field between the exposed surface of the coating 32 and the plate 30.

However, by inserting an auxiliary or second grid 36 between the first grid 34 and the target member 26 and maintaining it at a positive potential below the first crossover and/or breakdown potential, the potential of the exposed surface of the coating 32 may be limited to prevent the possible destruction of the coating 32. Further, it is particularly noted that this invention teaches that the auxiliary grid 36 is directly connected to the plate 30 by the connection 37. Thus, by controlling the potential source 39 which is applied to the plate 30, the potential applied to the auxiliary electrode 36 and to the exposed surface of the coating 32 may likewise be determined. It may be understood that the potential of the exposed surface of the coating 32 may not exceed the potential placed upon the second grid 36, which according to this inven tion is the same as the potential applied to the plate 30. Thus, the target member 26 does not have an opportunity to operate in the reverse field mode of operation since the exposed surface of the coating 32 is limited to the potential V of the plate 30 with the result that at this point no field is applied across the coating 32. Thus, in view of the teachings of this invention, it is impossible in the operation of the target member 26 to destroy the insulating coating 32 by an electric field wherein the exposed surface of the coating is driven positive with respect to the plate 30. It is noted however, that the potential source 39 which is applied to the plate 30 and the second gird 36 should not exceed that potential which would create a field across the coating 32 great enough to breakdown the coating 32 when the exposed surface is being charged to the potential of the cathode gun 16.

Thus, it may be seen that the target member of this invention will not breakdown due to an increasingly strong field being imposed across the insulating coating, because the field and the potential may be precisely controlled by the potential applied to the plate of the target member. Further, it is obvious that an additional potential supply has been eliminated by the teachings of this invention since the auxiliary or second grid may be tied directly to the same potential source as supplies the plate of the target member. In addition, the second or auxiliary grid may be spaced at extremely close distances to the surface of the insulating coating due to the absence of the need of providing insulating material between the grid and the surface of the insulating coating. As a result of the simplified physical mounting of the second electrode 36, the microphonics produced by the grid 36 may be substantially eliminated and as mentioned before the electrode may be spaced very closely to the surface of the insulating layer to thereby create a field of sufficient magnitude to permit adequate registration of the electron beam upon the target member. Finally, connecting the auxiliary grid to the plate of the target member permits using a closely spaced grid without providing a shunt capacity between the target member and ground.

While there has been shown and described what are presently considered to be the preferred embodiments of this invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. An electrical device comprising a target member including an element of elecrtically conductive material, and a layer of storage material disposed upon said element and having the property of generating electrons in response to an energy radiation representing a pattern of information to be recorded upon said target; and an electrically conductive grid spaced from an exposed surface of said layer, said grid being connected by a low impedance electrical path to said element to provide an electrical field across said layer of a value below the breakdown of said layer.

2. An image device comprising a target member including an element of electrically conductive material, and a storage layer disposed upon said element and having the property of generating electrons in response to electron bombardment to thereby establish a pattern of discrete charges upon said layer; means for directing a writing electron beam modulated with an input signal at said target member to generate secondary electrons within said layer and to establish a pattern of charges upon said layer in accordance with said input signal; means for directing a reading electron beam upon exposed surface of said layer to restore said exposed surface to an equilibrium potential while deriving an output signal corresponding to said input signal; and an electrically conducting grid disposed in a spaced relationship from said exposed surface of said layer, said conducting grid being electrically connected by a substantialy zero impedance connection to said element to establish such a potential upon said element to limit the electrical field established across said layer to a value below the breakdown of said layer.

3. An electrical device comprising a target member including an element made of an electrically conductive material, and a storage layer being disposed upon said element and having the property of generating electrons in response to an energy radiation representing a pattern of information to be recorded upon said layer in the form of discrete charges, said layer being subject to destruction when exposed to an electrical field above a given value, means for directing an electron beam upon an exposed surface of said layer to establish an equilibrium potential upon said exposed surface while deriving a signal corresponding to said pattern of information, a first grid disposed between said means and said target member for establishing a field to direct said electron beam normally upon said exposed surface; and a second grid disposed between said first grid and said target member, and being electrically connected by a substantially zero impedance path to said element to prevent the field established across said layer from exceeding said given value.

4. An electron image device comprising a target member including a backing element made of an electrically conductive material, and a porous layer of from one to five percent of the normal bulk density deposited on said plate and exhibiting the property of generating electrons within said porous layer in response to an electron bombardment, means for directing a first electron beam modulated with an input signal at said target to generate secondary electrons within said layer and to establish a pattern of charges upon said layer in accordance with said input signal; means for directing a second electron beam upon an exposed surface of said layer to restore said exposed surface to an equilibrium potential while deriving an output signal corresponding to said input signal; and an electrically conductive grid disposed in a spaced relationship with respect to said exposed surface, said conductive grid being electrically connected by a substantially zero impedance path to said element to prevent the electrical field established between said exposed surface of said layer and said conductive grid from exceeding the breakdown of said layer.

5. An electrical device comprising a target member including an element made of an electrically conductive material, and an insulating layer being disposed upon said element and having the property of generating electrons in response to energy radiations representing a pattern of information; means for directing an electron beam upon an exposed surface of said layer to derive a signal corresponding to said pattern of information; and an elec trically conductive grid disposed in a spaced relationship to said exposed surface at such a distance to provide a suflicient electrical field between said conducting grid and said exposed surface to insure the adequate resolution of said electron beam upon said surface, said conducting grid being electrically connected to said element to apply the same AC. and DC. potential to said element as to said conductive grid and to thereby prevent the electrical field established across said layer from exceeding a breakdown value.

6. An image device comprising a target member including an element made of an electrically conductive material, and a storage layer being disposed upon said element and having the property of generating electrons in response to an energy radiation representing a pattern of information to be recorded upon said layer in the form of discrete charges, said layer being subject to destruction when exposed to an electrical field above a given value, and a grid element spaced from said layer, said grid element being electrically connected by a low impedance path to said element to prevent the electrical field established between the surface of said layer and said element from exceeding said given value.

7. An image device comprising a target member including an element made of an electrically conductive material, and a storage layer being disposed upon said element and having the property of generating electrons in response to an energy radiation representing a pattern of information to be recorded upon said layer in the form of discrete charges, said layer being subject to destruction when exposed to an electrical field above a given value, a grid electrode, and a support member for positioning said grid electrode in a spaced relationship with said layer and for providing a substantial zero impedance path between said grid electrode and said element to prevent the electrical field established between the surface of said layer and said element from exceeding said given value.

8. An image device comprising a target member including an element made of an electrically conductive material, and a storage layer disposed upon said element and having the property of storing discrete charges in response to an energy radiation representing a pattern of information to be recorded, said storage layer being subject to destruction when exposed to an electrical field above a given value, means for directing an electron beam upon a surface of said layer to restore said surface to an equilibrium potential while deriving an output signal corresponding to said pattern of information, a grid electrode, and a support member for positioning said grid electrode in a spaced relationship with said layer and for providing a substantially zero impedance path between said grid electrode and said element to prevent said equilibrium voltage of said surface from establishing an electrical field across said layer in excess of said given value.

References Cited UNITED STATES PATENTS 2,929,866 3/1960 Melamed 3l5l1 X 3,242,367 3/1966 Szegho 313-89 3,277,334 10/ 1966 Toohig et a1. 315-11 X RODNEY D. BENNETT, Primary Examiner.

J. P. MORRIS, Assistant Examiner. 

